German Patent DE 197 34 695 C1 discloses a method for the correction of measuring errors of a coordinate measuring machine. A method for the self-calibration of the coordinate measuring machine is disclosed. Thereby, the coordinates of structures on a non calibrated reference object are measured in several rotational positions on the object table of the coordinate measuring machine. The measured coordinates are rotated back with rotating functions into the initial position. Thereby, a correction function is determined such that the coordinates rotated back have an optimal accordance with the coordinates of the initial orientation. Thereby, each reference object is rotated about only one angle. Rotation symmetrical linear combinations of the fit functions consulted to the approximation of the correction function are determined and waived during approximation. The generated correction functions are systematically complete and do not contain any indefinite or faulty terms.
High-precision coordinate measuring machines are used in the semiconductor industry for measuring structures on masks or wafers. The exact knowledge of the coordinates of the structures on masks is mandatorily necessary in order to conduct a controlled production of integrated circuits.
The measuring parameters of these high-precision coordinate measuring machines have error components which are dependant from the place of measuring that is the measured coordinate. Thereby, systematic error components exist, which result from the construction and the choice of the assembly parts of the coordinate measuring machine. Thus, for example known errors are to be found in the mirror orthogonality and the mirror planarity, in distortions in the scaling of the measuring axes (so called cosines errors) as well as in the deflection of the mask used for correction.
For reaching high-precision of the measurements, high-precision coordinate measuring machines need coordinate-dependant error correction. The determination of this correction is generally obtained by comparison with a standard. As for extremely high precisions which prevail for example in the measuring technique of semiconductor substrates, no adequate exact standard yet exists. Instead it is known to calibrate a coordinate measuring machine with itself by measuring one and the same object in several positions. With an error correction function generated by self-calibration all errors of the coordinate measuring machine are detected except for the scaling error. This scaling error can be detected only by comparison with a suitable length standard.
U.S. Pat. No. 4,583,298 describes the self-calibration of a coordinate measuring machine with a so called calibration plate onto which a grid is arranged. However the positions of the grid points are not calibrated. The grid plate is positioned onto the object table of the coordinate measuring machine and the positions of the grid points are measured. The same grid line is then being further rotated two or more times about 90° in each case about a rotation axis and the positions of the grid points are measured in each of the adjusted orientations. The measuring results are mathematically rotated back and different correction factors and tables optimized such that the data being rotated back is provided with a better accordance.
U.S. Pat. No. 4,583,298 deals in detail with the problems of faulty and unreliable corrections. Errors during the measurement of the measuring parameters consulted for the correction determination are determined as the source. It is shown that a mathematically definite correction is obtained only if more than two different rotational positions are measured with the same grid plate, and the rotational centers for the rotations between the rotational positions are thereby sufficiently different. For this purpose, the grid plate is positioned, as known, on the object table and the positions of its grid points are measured in several orientations of the grid plate. The orientations are obtained for example by several rotations of 90° about its center point. Afterwards, the grid plate has to be shifted, however, to a completely different position on the object table. There, the measurement of the position of its grid points is repeated in several orientations, as already mentioned beforehand. Thereby it is essential that the same grid plate must be shifted on the object table.
However, this requirement turns out to be not advantageous in practice, since the simplest way is to rotate the grid plate about such angles at which the outer dimensions merge. Thereby, the rotating point is always the center point of the grid plate. Thus, in U.S. Pat. No. 4,583,298 a square calibration plate is for example inserted in a square frame and positioned after each measurement shifted about 90° in said frame again. Therewith, all rotational centers are equal to the center point of the calibration plate. Only if the rotational centers are far apart that is if their spacings are akin to the spacings of the calibration structures, the error correction is better. But even if considerably different rotational centers are realized, the obtained correction factors as well as the correction result are not entirely satisfying.
In order to allow a significantly shift of the rotational centers, the holding mechanism such as the square frame must be shifted. For this purpose, also the measuring table must be enlarged in comparison with the not shifted object. The actions necessary for this conversion of the coordinate measuring machine are associated with significant drawbacks. Thus, a mounting of a shiftable holding frame for the calibration plate on the object table is problematic. If namely several mask holders are available on the object table (such as vacuum chuck or special more-point mounting), they would have to be mounted extra for the calibration measurement. The positioning of a holding frame on available mask holders is also out of question since they could be damaged and provide no plane positioning surface for the holding frame respectively.
Likewise, the enlargement of the measurement area for the measurement of the calibration plate in a shifted condition is problematic. Said enlargement requires cost intensive and constructive changes which are integrated into the production costs of the coordinate measuring machine. The overall dimensions of the coordinate measuring machine are also enlarged. However, the positioning area of the coordinate measuring machine affects directly the operating costs since the positioning area in the clean room is very expensive in the semiconductor industry.
U.S. Pat. No. 5,798,947 discloses a method, an apparatus and a computer program for self-calibrating two-dimensional tables for metrology. For this purpose, a rigid substrate is used, which has features on a regular grid in order to calibrate each of the two-dimensional table positions with respect to a coordinate system. A distortion function should in each case be determined for the X coordinate direction and the Y coordinate direction from the calibration. Thereby, each of the two-dimensional table positions of an array of table positions is connected to a Cartesian coordinate system in order to determine the distortions. Firstly, a substrate having a plurality of marks is positioned on the measuring table. The marks on the substrate are thereby spaced regularly from each other. Afterwards, each position of each mark is measured on the substrate. The substrate is thereby kept in an outlet reference position on the table. When this measurement is completed, the substrate is rotated about the reference position so that the substrate is transferred into a rotated reference position. Finally, the positions of the marks are measured on the substrate wherein the substrate is kept in the rotated reference position. A complete, non-four-fold rotationally symmetric distortion between the two-dimensional array of table positions and the Cartesian coordinate grid from the measured positions of the marks is determined in the original and the rotated reference position. The substrate is shifted about at least one interval (grid spacing of the marks) relatively to the original reference position. Therewith, the substrate is transferred into a shifted reference position. Afterwards, the positions of the marks on the substrate are measured, wherein the substrate is kept in the shifted reference position. An incomplete, non-four-fold rotationally symmetric distortion between the two-dimensional array of table positions and the Cartesian coordinate grid from the measured positions of the marks is determined in the original and the rotated reference position. Finally, a two-dimensional shifting error and a two-dimensional rotation error are determined from the complete, non-four-fold rotationally symmetric distortion and the incomplete, non-four-fold rotationally symmetric distortion. Likewise, a complete, non-four-fold rotationally symmetric distortion between the two-dimensional array of table positions and the Cartesian coordinate grid from the shifting errors and rotation errors, and the measured position of the marks is determined in the original, the rotated and the shifted reference position.
In the method disclosed in U.S. Pat. No. 5,798,947, special masks are used onto which the marks to be measured are arranged in a regular grid in the X and Y plane.
A coordinate measuring device and a method are known from the published patent application DE 10 2004 023 739. Thereby, a mask is also arranged on a measuring table which is shiftable into X coordinate direction and Y coordinate direction. A focusing optic and a detector are furthermore provided. The mask can be illuminated with a reflected light illumination device and/or a transmitted light illumination device so that the structure to be measured is imaged onto the detector.